Manny Pacquiao's trainer Freddie Roach admits the multi-weight world champion is in desperate need of a knockout against Juan Manuel Marquez to restore his reputation.

Pacquiao takes on the WBO light-welterweight champion for a fourth time on December 8 in Las Vegas after all three of their previous meetings - a draw and two Pacquiao wins - ended with contentious verdicts.

When the pair step into the ring in Las Vegas later this year, both boxers will be going for broke, looking for a knockout to avoid another disputable decision, which Roach believes will benefit Pacquiao.

"Marquez will be more hittable and not so much a counter-puncher. I think we need a knockout to secure [Pacquiao's] No. 1 spot in the world. He needs to be impressive and a knockout would be impressive," Roach was quoted as saying by boxingscene.com.

"In the first one [Pacquiao] was aggressive in the beginning of that fight. In the second fight we had one knockdown and in the third fight we had none. He just lost that killer instinct. In the first fight he was trying to knock him out, but Marquez is very resilient also.

"You have to take that factor in also. We need to bring back that killer instinct where he finishes guys like he used to."

Despite Marquez's protests that he deserved to win all three previous encounters, Roach was insistent that Pacquiao had done enough, but admitted a knockout would settle the matter once and for all.

"[Marquez] lost those fights and I still had Manny winning by a little bit. Manny started well and he finished well," Roach said. "A lot of fans thought that we lost [the third] fight so we have to redeem ourselves."